A coastguard spokeswoman says it would be a "miracle" if the five crewmen missing after a Norwegian anchor handling tug capsized yesterday 86 miles west of the Shetland Islands were found alive.
But the search is continuing for the five men, thought to be trapped inside the 75 metre Bourbon Dolphin. Ten of the 15 crew members were found after yesterday's incident. Seven are in hospital, but the ship's owners said three others had died.
Coastguard helicopter searches had been suspended until first light but Royal Navy divers continued to search throughout the night.
The coastguard said the search and rescue operation had changed to a search and recovery operation: "We are not really expecting to find survivors now. It would be a miracle if they had survived all night in five degrees, freezing water. It's very sad."
The coastguard confirmed that the ship was still attached to Transocean Rather rig by an anchor chain, and non-essential staff have been removed from the rig to Shetland as a safety precaution.
Jim Sinclair, watch commander at Shetland coastguard, said four helicopters and between six and seven surface craft had been involved in the operation.
He said the divers were conducting a survey and risk assessment of the ship using a remotely operated vessel (ROV).
"It's a very complex and delicate operation as the vessel is actually upside down and stability is uncertain so they want to make sure of their own safety before they enter the hull,” he said.
"She is connected and that's one of the reasons that the Transocean Rather was down manned just in case the vessel came up towards the rig on the tide change and damaged it."
Grampian Police said they would begin an investigation into the incident once the search and rescue operation was completed.
The ship, which is less than one year old, had been working in the vicinity of the Rosebank oilfield with a crew of 15.
RAF spokesman Michael Mulford told the BBC: "You realise that these guys, if they have managed to find that bubble of air, if they've managed to find just somewhere to keep themselves alive, the divers will get to them."
Trond Myklebust, manager director of Bourbon Norway, said: "There are divers at the scene and they have heat seeking cameras and there are also diving support vessels at the location."
He added: "The vessel is more or less brand new. It has been doing this operation since it was delivered.
"This is a common task and a common job in the whole area and it's absolutely incredible what has happened. We cannot understand it or believe it because this should not happen."
He said he could not speculate on what had caused the accident but said the vessel was carrying out an anchor handling operation with the rig and was connected to it.
The Bourbon Dolphin had a number of roles in the North Sea, including anchor handling and towing, the installation of subsea construction blocks and operations involving remote vehicles.
For more information visit www.mcga.gov.uk
